This invention deals generally with electrical measuring and testing, and more specifically with the determination of the non-electric property of the state of cure of concrete by the use of time domain reflectometry.
The degree of cure of concrete is of vital interest in the construction industry because the cure status determines the strength and degree of shrinkage of a concrete structure and therefore whether it can be subjected to subsequent construction processes. However, since the cure status of the internal portion of a concrete structure can not be evaluated visually, considerable effort has been expended to find methods to accurately evaluate the cure state concrete.
Several electrical methods to determine the cure state of concrete have been used. Among them are spin-echo nuclear magnetic resonance (U.S. Pat. No. 6,396,265 to Shtakelberg et al) and pulsed nuclear magnetic resonance spectrometry (U.S. Pat. No. 4,769,601 to Herrick). Miura et al have also performed laboratory tests (published in the Journal of the American Ceramic Society, Vol. 81, No. 1, pp 213-216) on the use of time domain reflectometry to evaluate the degree of concrete cure. However, Miura used a surface probe and took no measurements within the interior of any structures. Furthermore, the measurements were so widely separated in time, that the tests yielded little more than information the conditions of the initial and final states of the concrete.
The present invention has demonstrated a time domain reflectometry apparatus which furnishes highly accurate and very repeatable measurements of the free-water, bound-water, and ion-conductivity components of the chemical state of the curing concrete. The change in the relationships of these chemical states over a very wide frequency range can be used to closely follow the curing status of the concrete. The invention combines the simplicity of electrical sensing with miniaturization available from high frequency techniques in a time domain reflectometry measuring device. Furthermore, calibration readings have been established for many concrete mixes with specific additives, so that the apparatus is usable for in-situ monitoring of the cure status of most types of concrete.
The frequency spectrum of time delay reflectometry pulses in curing concrete separates into three easily identifiable components. One is the free water response representing water in the unattached state measured at the higher frequencies above 1 GHz. This quantity falls during the cure period. The second component is the bound water state representing water attaching to the developing microstructure, measures at the mid-frequencies between about 1 MHz and 1 GHz, the quantity of which increases during the cure. The third identifiable component is the ion conductivity, measured at the lover frequencies between about 10 kHz and 1 MHz, representing ions moving through the microstructure, which decrease during the cure.
Although it is feasible to observe these changes by transforming the data into a microwave frequency spectrum for scientific-quality analysis, direct interpretation in the time domain is far more useful for field use. A further benefit of the invention is that when the measurements are performed in the time domain, sensor response can be separated from other effects by propagation delay.
The invention is thereby very beneficial for overcoming quality control problems, increasing construction speed, and improving the uniformity of the concrete, especially in critical structures.
In the preferred embodiment of the invention, a sensor embedded within the structure being constructed receives and reflects a fast rise time pulse, and the reflected transient signal occurring in the microwave frequency range relates to the water states for any particular concrete mix. Because of this phenomenon, the reflected signal can be directly related to free and bound water states and percent of cure. In experimental tests, changes in the signal have been followed during processing and compared to other test methods to establish information in regard to the relationship of the reflected signal to the free and bound water states and to the state of cure.
The sensor is a miniature capacitor constructed at the end of a semi-rigid coaxial transmission line that is immersed in the curing concrete. Step function voltage pulses are fed to the transmission line, and the reflected signal from the transmission line is monitored. The amplitudes of the reflected pulse signals, which are related to the free water response, and the decay characteristics of the reflected pulse signals, which are related to the bound water, are then fed to a computer for interpretation and comparison to previously secured standards.
The invention thereby furnishes a real time measurement of the state of cure of the concrete, and the transmission line and sensor, which remain in place after the material is cured, can actually be used with the same signal generator and signal processing system to later check for cracks or discontinuities which might develop in the cured concrete at a later time.